This invention relates to an avalanche photodiode (APD) for use in optical communications, optical-information processing, optical measurement, etc., and more particularly to a staircase APD having good characteristics of low noise and high-speed response.
As photodiodes for use in the 1 to 1.6 .mu.m-band optical communications are known so far a pin photodiode comprising on a InP substrate, a lattice-matched In.sub.0.53 Ga.sub.0.47 As layer (referred to as InGaAs layer hereinafter) as a photoabsorption layer disclosed in the article Electronics Letters 20, 653-654 (1984), and an APD seen in IEEE Electron Device Letters 7, 257-258 (1986). Especially the latter is put in practical use for telecommunications because of its structure of providing internal gain due to avalanche multiplication action, and the consequent high sensitivity.
FIG. 1 shows a structural cross-section of a typical InGaAs APD. This device is formed by the following process: On an n-type InP substrate 1, an n-type InP buffer layer 2, and n-type InGaAs photoabsorption layer 3, an n-type InP avalanche multiplication layer 4 and an n-type InP cap layer 5 are fabricated on top of one another by the vapor phase epitaxy (VPE) technique. Following this, a p.sup.+ region 6 is formed, for example, by thermal diffusion of Zn, and the p-type guardring regions 7 are formed by beryllium-ion implantation followed by thermal treatment. Over the whole surface a passivation film 8 is deposited. Finally an n-side ohmic electrode 9 and a p-side ohmic electrode 10 are deposited by the vacuum evaporation, thus a completed device being obtained.
The operation of the APD will be described. When light 11 is incident on the surface, charge carriers are created in InGaAs light-absorption layer 3. In particular, holes are injected into InP avalanche layer 4 under electric field. In InP avalanche layer 4 to which high field is applied, ionization by collision occurs, which results in carrier multiplication. It is known that this carrier multiplication process due to the ionization resulting from the random collision of carriers governs important characteristics of the device: low noise and fast response. In other words, a greater difference between ionization rates of electrons and holes in the avalanche layer reflects higher ratios of the ionization rates. (Let .alpha. and .beta. be ionization rates of electrons and holes respectively. If .alpha./.beta.&gt;1, electron will be the majority carrier that causes ionization by collision and if .alpha./.beta.&gt;1, hole will be so). This is desirable for the characteristics of the device. The ionization rate ratio .alpha./.beta. or .beta./.alpha.) however depends on the properties of the material, for example, .beta./.alpha..congruent.at most about 2 for InP which differs much from the .alpha./.beta..congruent.20 for Si having a low-noise advantage. Neco approaches using epoch-making material therefore are required to realize characteristics of lower noise and higher-speed response.
For such purpose, F. Capasso et. al. proposed a staircase APD permitting higher-sensitivity and higher bandwidth due to an increase of the ionization rate ratio .alpha./.beta. by utilizing the conduction-band discontinuity energy (.DELTA.[O]Ec) to enhance the electron ionization. Such an example is described in the article IEEE TRANSACTIONS ON ELECTRON DEVICES ED-30, 381 (1983).
In this staircase APD, the value of the conduction-band discontinuity energy (.DELTA.[O]Ec) contributes greatly to improvement in the ionization rate ratio. Additionally it is constructed to make it possible to solve the electron pile-up at the hetero-interfaces of the multiplication layer, associated with the superlattice APD with a multiplication layer of rectangular superlattice structure disclosed in the article Appl. Phys. Lett. 57, 1895 (1990).
In the staircase APD proposed by F. Capasso et al., however, the periodic structure of the layer graded in composition from InGa.sub.x Al.sub.(1-x) As to InGaAs is used as an avalanche layer. When a high electric field (&gt;500 kV/cm) is applied therefore, the tunneling dark current increases at the minimum bandgap of 0.75 eV (of InGaAs), and in turn, the dark current providing a multiplication factor of 10 is estimated to get above 1 .mu.A. The bandgap of such an InGaAs layer allows a large dark current to flow, and this can cause a large power penalty in optical communication so as to make it practically impossible to use, thereby constituting a disadvantage.
By the way, the example proposed by F. Capasso et. al. is not enough by itself for obtaining an APD that is practically usable for optical communication, and hence it is needed to separate the photoabsorption layer and the avalanche layer like the above-mentioned InGaAs-APD example, and to control the electric field strengths impressed on the layers to suitable values to share the function for the above-mentioned purpose by them. Such an example of the above-mentioned superlattice APD was reported in the article Appl. Phys. Lett. 57, p.1895 (1990). FIG. 2 shows the structure of this superlattice APD with separate photoabsorption and avalanche layers, together with the distribution of electric field strength. Reference characters in FIG. 2 designate as follows: 1 and n.sup.+ -type InP substrate, 2 and n.sup.+ -type InP buffer layer, 12 an n.sup.+ -type InAlAs Layer, 14 an p-type InGaAs electric field relaxation layer, 17 a p.sup.- -type InGaAs photoabsorption layer, 18 a p.sup.+ -InAlAs window layer, 19 p.sup.+ -type InGaAs ohmic contact layers, and 20 an InGaAs/InAlAs superlattice avalanche multiplication layer.
For the purpose of ensuring a sufficiently-large electric field strength (&gt;400 kV/cm) to induce ionization in superlattice avalanche multiplication layer 20, and a small electric field strength (&lt;150 kV/cm) to drift photogenerated carriers and prevent tunnel breakdown in photoabsorption layer 17, there is provided a structure with a p-type InGaAs electric-field relaxation layer 14 sandwiched between both layers 20 and 27.
This structure has the disadvantage that because of field relaxation layer 14 having the same composition InGaAs as that of the light-absorbing layer 17, tunnel breakdown may occur due to the high field in the electric-field relaxation layer, resulting in increased dark current. Additionally in practical photodiode-fabrication processes, diffusion of a dopant may take place from the highly-doped electric-field relaxation layer to the absorbing layer side and the avalache multiplication layer side. The former may be a cause of inducing tunnel breakdown and the latter may be a cause of failing to obtain the electric field as high as designed, with a consequent inadequate multiplication.